Kushalas Attacks are all based on wind. Even by simply approaching him , you will get blown away by his wind-aura. So what you want to do is break the horns and spikes on his head. Once you’ve done so, the wind-aura is gone. Using a thunder or dragon element longsword is favorable due to its flexibility. If you feel more comfortable with other weapons, go ahead. The easiest way to land some hits is by waiting for Kushala to stay still and spitting witting-blasts. During that attack he is completely defenseless and it’s a perfect opportunity to beat the horns from his head. Poison is another useful element as Daora will lose its wind-aura during the period he is poisoned. However this will rob you of the possibility to gain elder dragonblood from it (Kushala being a dragon elder) as the blood becomes polluted.

Flashbombs are very useful when Daora jumps and starts doing the windstream. By simply throwing a Flahbomb somewhere in front of him, he will fall to the ground and remained confused for a while, which gives you plenty of time to attack it.